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You can only create a maximum of 3 ad units and 3 link units on your page.

According to Google:

Certain locations tend to be more successful than others. The "heat map" picture below illustrates the ideal placing on a sample page layout. The colors fade from dark orange (strongest performance) to light yellow (weakest performance). All other things being equal, ads located above the fold tend to perform better than those below the fold. Ads placed near rich content and navigational aids usually do well because users are focused on those areas of a page.

You should also read the following links one after the other to learn all about adsense placement including some very solid tips that work: